THE COMPOSITION OF THE EARTH 



51 



amples. If the sedimentary beds were broken and the broken 

 edges displaced (faulted), the intrusion is a bysmalith. The 

 Spanish Peaks of southeastern 

 Colorado are an illustration. 

 Deep-seated intrusions of very 

 great size (often many miles 

 across) are known as batholiths. 

 Usually batholiths are of ir- 

 regular form. Unlike laccoliths, 



they do not simply bulge the FIG. 28. Diagram of a laccolith 



cover, but occupy vast spaces with associated dikes and sills - 

 which have been actually hollowed out of the preexisting 

 rocks. Whether this was accomplished by melting and as- 



FIG. 29. Two Buttes, Prowers County, Colo. Sandstone beds uplifted by 

 a laccolithic intrusion. The slopes have been modified by erosion. (Dar- 

 ton, U.S. Geol. Surv.) 



similating the previous rocks, or otherwise, is not known 

 definitely. Such intrusions are of rather common occurrence 



in eastern Canada, in asso- 

 ciation with very ancient 

 rocks. Erosion has removed 

 their original covering, ex- 

 posing the igneous cores. 

 Granite batholiths form the 



. 30. Diagram of a stock. , . ,. ., 



central cores of many of the 

 great mountain ranges. Certain bodies of intrusive rock, ex- 

 posed by erosion and rudely circular or elliptical in ground 

 plan, are called stocks (Fig. 30). They vary in diameter at the 



